Mechanical Engineers look here!! (cutting springs)

[allgripnoslip]

I understand the negative affects of cutting stock springs, but it seems like its due to ignorance. I want to do it the logical way. Cutting the springs to gain a more advantageous rate with lowering as a bonus.

I did a little bit of searching and found some spring rates of aftermarket and stock springs:

H&R Race Spring
Front-430 lb/in
Rear-260 lb/in

Stock 97-99 awd spring
Front-246 lb/in
Rear-151 lb/in

Is there some type of formula (eg. hooke's law) where I can calculate how much the spring rate will change when coils are removed from it? or would you have to actually test and find out...
Id like to cut the springs enough to get close to the rates of the H&R springs while not going any lower than 2"-1.8".

Perhaps if I can figure out a way to do it I can do a write of with a table of values and show that if you cut your springs a given amount, it will lower the car X amount and raise the spring rate to Y amount.

Any help would be appreciated!



* How many coils does a stock 2g spring have and is it the same or different from front to rear*

 

[drivemusicnow]

go to this site:

http://www.engineersedge.com/spring_comp_calc_k.htm

here is what you have to do, use this, and the other spring calc on that site.

First take a your cars springs figure out how many active coils each spring has, it standing height, and compressed height. Then you have to figure out how many lbs force it takes to compress the spring to that "compressed height"

This will calculate out to give you the modulus. Then use the other calculator and mess around with how many active coils you have to determine the spring rate you want.

I know that heat will change the modulus of the metal, and I'm also not sure if cutting a coil out is the same as haveing a non active coil. It should be.

 

[allgripnoslip]

I think I may have answered some of my own questions, however It would still be great for others to post thier opinions/experiences

I found this information from a website:
"Imagine a 10 coil spring sitting upright on a table. You place 200 lbs on the spring. The weight moves the top of the spring down 1 inch. So your spring rate is 200 lbs per inch. Each one of the ten coils has shrunk 1/10 of an inch.

Now cut the spring in half. You have 5 coils now. When you place the 200 lbs on it, each coil still shrinks 1/10 of an inch. (same diameter coil, same amount of deflection.) Because there's only 5 coils, the spring will only move down 5/10 of an inch.

This gives you a new spring rate of 400 lbs per inch. Twice as short = twice as stiff."

I searched again and found some 2g coil springs and counted the coils.
7 in the rear and 5 up front unless I counted wrong because it was hard to tell from the pics.

with 7 coils rated at 151 lb/in in the rear and 5 coils rated at 246 up front

cutting one coil from the front and rear gives the following rates:

151 * (7/6)= 176.17 lb/in rear
246 * (5/4)= 307.5 lb/in front

use this formula to calculate the new spring rate after removing a spring

original spring rate * (original coils/ original coils-cut coils) = new spring rate

ex. 151 * ( 7/ 7-1)= 151 * (7/6) = 176.17 lb/in

Cutting 2 coils
151 * (7/5)= 211.4 lb/in
246 * (5/3)= 410 lb/in

cutting two coils gives spring rates very close to the H&R race springs, however the ride height from cutting two springs may not be desireable, I really dont know how much cutting coils lowers the car, if anyone could comment on that information that would be awsome.

 

[allgripnoslip]

It took me over an hour to write that last post and before I posted noone had posted yet. I wonder how much my information will change taking those other factors into consideration. I was looking for somewhat of a rough estimate. I currently dont have the information require to make those calculations that you've shown me with out removing a spring directly from my car and measuring it myself. SO basically I have to find the wire diameter and mean diameter of the coil, the unloaded height of the coil, and then that height minus one inch and use the given spring rate to find the modulus and then figure out which coils are active and which arent. I wish I had a set of these coils to do the measuring.

 

[SAKONE]

You know what. I have factory GSX springs in my backyard being used as plant holders. I'll update this post if you want to know or take pics if no one else has posted anything by then. It's dark out and don't feel like walking through soggy grass and getting a flashlight b/c I'm lazy like that. I'll wait till after work tomorrow to post if you can wait.

 

[allgripnoslip]

Thanks ! that would be great, Im leaving tomorrow and wont be back until sunday so I shall post a reply if you do get those pictures up.

 

[allgripnoslip]

Anyone have anything else to add? If I end up doing any suspension work its probably going to be done right with some Tein coilovers. I recieved this PM from a new user who had this little bit of information to add

I am a mechanical engineer, but I am considered a newbie on the forum so I can't post a reply in the forum. No matter what the calculations tell you, the biggest problem is going to be the heat of cutting the coils. You have no way of accurately measuring the heat exactly for each spring as it is being cut. This will change the spring rate for each spring differently. If it changes it too much, it could drastically and dangerously affect your handling. If you had the spring rate checked after the cut to verify your calculations and they are close enough to what you calculated, they should be safe. If it doesn't work, I hope you have a back up plan once you cut them.

I hope this helps,
Dennis

 

[drivemusicnow]

The heat factor that Dennis stated is truly one of the biggest reasons why cutting coils is so looked down upon. the more you heat the metal, typically, you will significantly lower the modulus. which means a much softer spring.

I wonder if you could get giant bolt cutters to work?


For the record I am not telling you to actually do this.

 

[allgripnoslip]

I was thinking about how machinists squirt special coolant while they are cutting certain materials, but Im not sure if it would dissipate enough heat. the bolt cutter idea sounds interesting.

 

[TurboB5]

sawzall?

this is really interesting actually...wheres the engineers at i love learning about this

 

[TimG]

I'm almost positive that the springs, no matter how many coils, keeps the constant spring rate. The force applied on the spring is the exact opposite of the spring upward force when at equillibrium.

The sumation of the forces along the strut axis (we'll call it the y- direction) is equal to zero. That being established, the forces exerted on the spring is = m(g). (Mass x Accel due to gravity). There is still the same amount of force being pushed down to the spring weather it's cut or not.

The forces pushing back are = k(x), where k is the spring constant and x is the compression or deflection of the spring. Compression in this case. The spring will still deflect the same distance when you cut it, it will just start out a hell of a lot shorter. (ie - 10"-8"=2" and 7"-5"=2". Both equal to 2" but one is lower.

The spring constant should not change when cut or lengthened (if thats even possible). Therefor, your car will sit lower but will not be any stronger with regard to the load put on it. It will only push back with the same amount of force that the car has sitting on it, while sitting lower to the ground.

In short, $200 is a good investment to buy lowering springs.

Anyone can chime in if they want. It's ben a while since I've looked at a spring equations.

EDIT: ARG, I just read the spring calculator site and I'm wrong. When you cut a coil off the spring, the k value (spring constant) goes higher. This means when you cut a coil, the spring will not compress as much as with the extra coil on there. BUT, you are starting out at a coil shorter. So you cut a coil to have it sit lower but the spring doesn't compress as much as it used too. Neat to know but I still vote for buying springs from Eibach or H&R.

 

[drivemusicnow]

TurboB5: what else do you want to know? The spring calculator I posted does anything you want.

TimG: Honestly, when I first read the post I had a similar reaction as you... then I thought about it and figured it out. Then I found a pre-made calculator.

Confusing concept but anywho.


By the way, I doubt there are too many MEs around here. The only "engineers" i think you see are the engineering students (like myself). Once we get jobs we all move on to nicer, more reliable, and faster cars just kidding.

 

[wret]

the biggest problem is going to be the heat of cutting the coils..

Nah. With an angle grinder cutting wheel or similar, the coil will be off before the rest of the spring knows what's going on. A sawzall would take forever and a torch might cause problems.

If you are really that concerned. Stop every few seconds to cool things down.

 

[Talesin]

Of course with the cut springs you'll also get wonky camber/caster, unless you're extremely careful about the angle you cut them at. The angle at which they rest on the spring perches is relatively constant with the way the OEM springs are made. Cut, that becomes a variable. You don't want your springs to shift around mid-cornering after all, and have the car suddenly drop an inch, screwing up the traction circle.
One more reason to laugh at Honda-kids that cut their springs to ghetto-lower their car.

Just shell out for actual lowering springs. Do it right, buy it once. The most important lesson my car has taught me.

 

[suicidal2af]

Of course with the cut springs you'll also get wonky camber/caster, unless you're extremely careful about the angle you cut them at. The angle at which they rest on the spring perches is relatively constant with the way the OEM springs are made. Cut, that becomes a variable. You don't want your springs to shift around mid-cornering after all, and have the car suddenly drop an inch, screwing up the traction circle.
One more reason to laugh at Honda-kids that cut their springs to ghetto-lower their car.

Just shell out for actual lowering springs. Do it right, buy it once. The most important lesson my car has taught me.

Cutting springs won't affect caster at all. Caster is a measurement of the angle between a car's steering axis and a vertical line through the center of the hub. Cutting the springs will affect camber, but no more than a set of lowering springs. The lower balljoints don't move when you drop a car, but the upper balljoint will shift downwards. Whether a spring is cut or manufactured at a certain rideheight, equal lengths/rates will give an equal camber.

It would be very easy to match the angle it sits on the perch. And the springs, once settled, won't shift around at all.

Lowering springs aren't doing it right.

 

[evi1clipse]

does any body know what type of metal is used for the springs that way you'll actually figure the best way to cut it.

 

[mavisky]

The coils are steel, a sawzall or cut-off wheel wouldn't heat the coils past normal temps more than an inch or two away from your cut by the time you were done making the cut.

I've come to the realization lately that the suspension is the last place I really want to skimp on.

 

[jtmcinder]

The lower balljoints don't move when you drop a car, but the upper balljoint will shift downwards.

And here I was, thinking the upper and lower balljoints were held a fixed distance apart due to the fact they are on opposite ends of a solid piece of metal (known as the knuckle). Where do I get these ideas?

Just cut one coil and be done with it.

- Jtoby

 

[SAKONE]

Forgot about this thread since the whole "downtime" this place had. Here's some OEM goodness GSX spring pics.

Here's a pic of either the front or rear.

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Here's what I grow in my garden.

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Using my spring to measure the tape.

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I'll paint these green and sell them as TEINs.

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one on one action.

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Springs in spring, is this what you sorta needed?

 

[drivemusicnow]

I hope the coils that are smaller (in diameter) are all "dead" coils. otherwise this becomes a MUCH harder math problem.

That last picture just gave me that, mostly bad, idea to have someone thread a thinner spring inside the stock springs... thus DUAL SUSPENSION SPRINGS!!! like valve springs only cooler... cause they're bigger.

 

[jtmcinder]

They don't all start "dead," but they are the first to go. That's how/why the stock springs are progressive.

- Jtoby

 

[snox135]

I'm almost positive that the springs, no matter how many coils, keeps the constant spring rate. The force applied on the spring is the exact opposite of the spring upward force when at equillibrium.

No there are progressive, dual and triple rate springs out there. I'm not sure what the stock springs are.

 

[suicidal2af]

And here I was, thinking the upper and lower balljoints were held a fixed distance apart due to the fact they are on opposite ends of a solid piece of metal (known as the knuckle). Where do I get these ideas?

Just cut one coil and be done with it.

- Jtoby

I meant shift inwards. I was tired and made a mistake, no need to be a sarcastic idiot about it.

 

[allgripnoslip]

NEW INFO

http://www.roadraceengineering.com/eclipsetech/springrateinfo.htm

Ill be doing calculations and post back soon.

 

[allgripnoslip]

Could someone take some more measurements? Im looking for the mean coil diameter(inches) and the diameter of the wire? Thanks